1. Field of the Invention
The present invention relates to an active matrix liquid crystal display apparatus having an array of liquid crystal pixels driven by an array of thin film transistor (TFT) switching elements which are selected on a row by row basis by scanning signals applied to row electrodes (scanning electrodes) and which have display data signals applied thereto via column electrodes (signal electrodes) to control the switching elements in a selected row.
2. Description of the Prior Art
The active matrix liquid crystal display apparatus is known. The apparatus basically includes upper and lower substrates having sandwiched therebetween a liquid crystal layer. Provided on the lower substrate are signal bus lines arranged in columns, scanning bus lines arranged in rows, pixel electrodes arranged in an area between two adjacent signal bus lines and two adjacent scanning bus lines to form a liquid crystal pixel, and, as active switching elements, thin film transistors (TFTs) respectively connected to a corresponding pixel electrode, a corresponding signal bus line and a corresponding scanning bus line. Provided on the upper substrate is a common counter electrode.
The apparatus is operated by applying a scanning voltage to the scanning bus liens on a row by row basis while applying display data signal voltages to the signal bus lines for the display of data of the pixel electrodes in a scanned row. The voltage applied to the pixel electrode (applied across the pixel electrode and the common counter electrode) changes or maintains the optical state (bright, dark, grey) of the liquid crystal between the pixel electrode and the common counter electrode to effect a display corresponding to the display data. The pixel electrode and the common counter electrode form a capacitor for holding the applied voltage to maintain the optical state during one scanning period.
In such an active matrix liquid crystal display apparatus, it has been proposed to form auxiliary capacitors each being parallel to the capacitor formed by a pixel electrode and the common counter electrode so as to improve the voltage holding characteristic of each pixel. A conventional method for forming the auxiliary capacitor by providing a common electrode below the pixel electrode, gate bus line and source bus line is disclosed by S. Hotta et al. in a paper title "Full-Color Multi-Gap LC-TV Display Panel Addressed by a-Si TFTs", SID 86 DIGEST, No. 16-6, pp. 296-297. In this method, however, when the display density of the display apparatus is increased (i.e., when the number of pixels per unit area is increased by reducing the sizes of the pixel electrodes, signal and scanning bus lines and TFTs), the voltage of each pixel will be affected by an electric field leaked from the signal and scanning bus lines, whereby the display quality will be deteriorated.